[okular] [Bug 491243] New: Buggy "smooth scrolling" behaviour during initial load of a PDF

[Stephan Sokolow]

https://bugs.kde.org/show_bug.cgi?id=491243

            Bug ID: 491243
           Summary: Buggy "smooth scrolling" behaviour during initial load
                    of a PDF
    Classification: Applications
           Product: okular
           Version: 24.05.2
          Platform: Flatpak
                OS: Linux
            Status: REPORTED
          Severity: normal
          Priority: NOR
         Component: general
          Assignee: okular-devel at kde.org
          Reporter: kde_bugzilla_2 at ssokolow.com
  Target Milestone: ---

SUMMARY

With smooth scrolling disabled, when I open a PDF in Okular, for the first
[variable amount of time] before the document "stabilizes", I have to deal with
pages slowly sliding upward by several lines of text from where I want them to
be once I finish spinning the scroll wheel.

As a programmer with no experience with the Okular codebase, my intuition is
saying "Is there some kind of scroll-position race condition that happens if I
scroll while the document is still loading/rendering something, which is then
getting eased via some bit of smooth scrolling code that ignores the
enable/disable checkbox?"

STEPS TO REPRODUCE

1. Uncheck Appearance → Use Smooth Scrolling (to optimize for perceived
snappiness instead of bling)
2. Load a long PDF like the electronic copy of Programming Starter Kit of
Macintosh that was included on various vintage CodeWarrior Reference CDs. (860
pages. The fact that it's not just a package of JPEG page scans may also be
relevant.)
2. Start scrolling as soon as pages start to render in (so you can see what
you're doing) but without waiting for the whole document's page thumbnails to
render or whatever else Okular is doing that takes a while.

OBSERVED RESULT

Whether scrolling up or down, when I stop spinning the scroll wheel, the
viewport slowly "slides down" (i.e. the document slides up) by three to five
rows of text from where I tried to put it.

EXPECTED RESULT

As soon as the scroll wheel stops being spun, the scrolling should stop and
stay there.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Kubuntu 22.04.4 LTS
KDE Plasma Version: 5.24.7 (Host)
KDE Frameworks Version: 5.92.0 (Host) / 6.4.0 (Okular Flatpak)
Qt Version: 5.15.3 (Host) / 6.7.2 (Okular Flatpak)

ADDITIONAL INFORMATION

I know that FocusWriter had a "bug" in its theme selector that turned out to be
"Qt 6 defaults to single-click-to-activate and listens to different settings
than the ones Plasma 5.x System Settings sets", so, for all I know, this
*could* be another "Qt 6 on KDE 5" bug... but it seems more likely this is just
"Disabling smooth scrolling doesn't get enough testing".

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